It has long been desirable to employ high speed centrifugal pumps as fuel pumps in aircraft employing turbine engines. Fuels used in aircraft turbine engines are typically of low lubricity. Centrifugal pumps are ideally suited for pumping low lubricity liquids without excessive wear, thus providing one reason for the desirability as an aircraft fuel pump.
Further, in many cases, for a given pumping capacity at rated engine speed, a centrifugal pump will occupy a considerably lesser volume than a typical positive displacement pump used for the same purpose. And this reduction in volume translates into a weight savings as well.
In aircraft, a reduction of the size of the envelope occupied by a given component provides the aircraft designer with greater flexibility in achieving an aerodynamically slippery design. Consequently, a smaller envelope made possible by reduced volume raises the potential for the more efficient operation of aircraft through the reduction in drag.
At the same time, the accompanying weight reduction enables aircraft range to be increased. The weight carrying capability of the aircraft heretofore devoted to transporting a positive displacement fuel pump can, in part, be used to increase fuel carrying capacity and/or other pay loads.
Notwithstanding the foregoing, centrifugal pumps as fuel pumps in turbine powered aircraft have not yet achieved an appreciable degree of utilization for the purpose. Conventional centrifugal volute pumps do not have the ability to provide fuel flow at high pressure at low engine speeds, particularly during engine starting sequences. To overcome this difficulty, it has been proposed to utilize positive displacement pumps in combination with centrifugal pumps with various decoupling schemes. A positive displacement pump is utilized to provide fuel flow during low engine speeds as during start up and when a suitable engine speed has been attained, the positive displacement pump is decoupled from the engine and the pumping operation assumed entirely by a centrifugal pump, typically of the conventional volute type.
While this approach appears sound in theory, in practice, during the transition from a low speed or a start operation to normal speed operation has been troublesome. A pressure instability will exist at the transition and that in turn can result in a disturbance in the fuel flow to the engine. The disturbance in fuel flow raises the possibility of an engine flame out.
The present invention is directed to overcoming one or more of the above problems.